UrPER CHALK. 
89 
chalk-pit, and some other localities, have produced 
similar exam])les. 
Obtuse rhomboidal crystals, of great beauty, 
have been found in a chalk-pit near Alfriston : 
their colour is of a delicate pearl white, and in 
their general appearance they resemble the double- 
retracting spar of Iceland, except in their inferior 
degree of transparency. The cavities of echinites 
are sometimes lined with rhomboidal crystals of 
carbonate of lime, disposed in lines parallel with 
the sections formed by the arem of the shell ; and 
the inner surfaces of the terebratuhc are frecpiently 
frosted over with drusy crystals of the same sub- 
stance. 
4. 8ul})huret of iron, or iron pyrites, in sub- 
globidar and irregular masses, is very common in 
the upper chalk. The external surface of the 
specimens is invested by crystals of a pyramidal, 
octaedral, or cubo-octaedral form ; and their in- 
terior exhibits a radiated structure, possessing a 
brilliant metallic lustre. When broken, and ex- 
posed to the action of air and moisture, they un- 
dergo decomposition with great rapidity ; and 
even in cabinets, frequently form an efflorescent 
sulphate of iron, and crumble into dust. This 
mineral occasionally encloses flints, shells, echinites, 
kc.y and frequently fills up the cavities of the 
latter. A specimen in my possession exhibits, on 
the upper side, a sharp cast of the interior of a 
spatangus ; and its base is covered with an elegant 
group of quadrangular pyramids, evidently the 
terminations of octatidrons, with their inferior 
angles concealed. 
